Alfalfa

Anthracnose

Pathogen:Colletotrichum trifolii

SYMPTOMS AND SIGNS

Anthracnose can affect leaves and stems, but crown rot is the most
important phase of the disease. The most obvious symptom is the bluish-black,
V-shaped rot in the crown.
Dead stems associated with such crowns are sometimes bleached white. Because
stems die suddenly, the dead leaves do not drop from the stem.

Anthracnose also causes
small, irregularly shaped blackened
areas on stems that become large, oval, or diamond-shaped straw-colored lesions with black
borders. Black fruiting bodies (acervuli), which under a hand lens look like
small dots, develop in the lesion. As lesions enlarge, they may coalesce,
girdle, and kill affected stems. In summer and fall, dead shoots (white in
color) are scattered throughout the field.

COMMENTS ON THE DISEASE

Anthracnose is a common problem in older alfalfa stands. The fungus
persists in alfalfa debris and crowns. The disease reaches maximum severity
during late summer and early fall coincident with warm and humid weather.
During the growing season, spores on stem lesions are a source of inoculum.
Splashing rain and irrigation water disperse spores onto growing stems and
petioles. Spores may also be spread with seed contaminated during the threshing
process.

MANAGEMENT

Control of anthracnose involves use of resistant cultivars or
cultural practices. Start looking for signs of anthracnose in early summer. If
an infestation is found, harvest alfalfa before losses become too severe.

Rotation with crops
other than clover and alfalfa for at least two years will eliminate sources of
inoculum in the field. For
more information, see CROP ROTATION.

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